Terminology

A paste-like substance specifically engineered to be applied at an interface between a heated surface and the surface of a heat sink. It aids or speeds up the removal of heat from original surface (device) to another (heat sink). The device being cooled has a more efficient operation and the useful lifetime is increased.

Viscosity:

A measure of the resistance of a fluid-like material (such as thermal paste) to deformation by either a shear or tensile stress. It can be thought of as the “thickness” or “internal friction” of a compound to flow.

Rheology:

The description of the flow characteristics of these highly filled paste-like solids in response to an applied force. They tend to be Newtonian or thixotropic in nature.

Newtonian Flow:

Material flow is determined by only its viscosity at a given temperature and tends to be independent of the shear force applied. The viscosity does not change in response to an applied force (stress). Examples of this behavior are honey and higher molecular weight silicone fluids. Newtonian fluids can be characterized by a single coefficient of viscosity for a specific temperature.

Thixotropic:

A material’s flow is very much dependent on the shear or tensile stress applied. The coefficient viscosity tends to decrease as the applied force is increased. Examples of this behavior are toothpaste, peanut butter, and mayonnaise.

Bond Line:

The limiting or minimum thickness that an interface material where a minimum thermal resistance is achieved. The thermal resistance will not change at thicknesses less than this value.

Dielectric Strength:

Dielectric strength is defined as the maximum voltage required to produce a dielectric breakdown (electric current flow) though the material. It is expressed as Volts per unit thickness. The higher the dielectric strength of a material, the less able it is to conduct an electrical current. A common accepted measurement technique is the ASTM D 149 method.

Electrically Conductive:

Sometimes is referred to as specific conductance. It is a measure of a material’s ability to conduct an electric current. Most pastes are magnitudes less electrically conductive than metallic copper or aluminum wire. Pastes containing ceramics and metal oxides are electrically non-conductive. Even those containing metallic particles are essentially electrically non-conductive compared to copper wires and vias. Problems arise when metal particles containing pastes break down and particles migrate onto the surfaces of devices causing short circuits.

Outgassing:

Refers to trapped gas being released from a solid when it is heated and/or placed under a vacuum. Typically, this is an important property for silicone fluid based thermal compounds. The volatile components released with many silicone fluids tend to cloud optical surfaces. This is much less prevalent with non-silicone fluid-based materials. Outgassing is often tested by the ASTM E595 method. This test consists of an application of a vacuum of 10-5 Torr. at 100 oC, for 24 hours. The collected volatile condensed material (CVCM) and the total mass loss (TML) are measured using this method.

Units of Thermal Conductivity:

Currently given in W/m-K (Watts/meter-degree Kelvin). Older literature values may be reported in cal/cm2)/sec/cm/oC. For example, copper has a value of 0.94 cal/(cm2)/sec/cm/oC = 394 W-m/K.

Pump Out:

This is a phenomenon that occurs with lower viscosity materials, such as thermal greases, where the interface material appears to ooze or migrate out from between two surfaces after a period of use. It is believed to occur because the material is successively stressed mechanically due to changes in temperature. As a result, the two surfaces expand and contract at different rates; stressing the material and causing component separation. Small mechanical deformations occur when electrical current passes through the device.